1. Field of Invention
The invention relates to a vehicle hybrid driving apparatus that has an engine and an electric motor as drive power sources, and particularly concerns size reduction of the power transmission device, and improvement in the fuel economy thereof.
2. Description of Related Art
There is known a driving apparatus for a hybrid vehicle whose drive power sources are an engine that is an internal combustion engine such as a gasoline engine, a diesel engine, etc., and an electric motor that is driven by electric energy. An example of such an apparatus is a hybrid driving apparatus (power transmission apparatus) described in Japanese Patent Application Publication No. 6-169504 (JP-A-6-169504). In the hybrid driving apparatus described in the publication No. 6-169504 (JP-A-6-169504), an output of a torque converter (fluid transmission device) linked to an engine (internal combustion engine) is linked to an intermediate shaft via an input clutch, and an electric motor is linked to the intermediate shaft so that power can be transmitted, and an input shaft of an automatic transmission is also linked to the intermediate shaft. During an engine mode, the input clutch is engaged, so that power of the engine is transmitted to driving wheels via the torque converter and the automatic transmission. During an electric motor mode, the input clutch is released, so that output of the electric motor is transmitted to the driving wheels via the automatic transmission. Due to the foregoing construction, during the engine mode, since power of the engine is transmitted via the torque converter, smooth starting and acceleration of the vehicle is maintained. Besides, during the electric motor mode, since power of the electric motor is transmitted without the intervention of the torque converter, the power loss by the torque converter is eliminated, and therefore relatively long cruising distance can be achieved.
However, in the hybrid driving apparatus (power transmission apparatus) described in the publication No. 6-169504 (JP-A-6-169504), even during the electric motor-driven travel, the automatic transmission sometimes needs supply of hydraulic pressure, and therefore the engine is operated at a certain rotation speed, with the input clutch being in the released state, in order to drive a mechanical type hydraulic pressure pump that acts together with the driving of the engine. Therefore, since the engine is driven even during the electric motor-driven travel, the fuel economy declines. To overcome this problem, the foregoing publication indicates that an electric oil pump is provided, and during the electric motor-driven travel, the electric oil pump is driven and the engine is stopped. However, this has a problem of rise in the manufacturing cost; for example, it becomes necessary to provide a control device that carries out a control of switching the on and off-states of the electric oil pump and the electric oil pump. Besides, there occurs another problem of size increase of the apparatus due to the mounting of the electric oil pump. Furthermore, there arises a need to improve the reliability of the electric oil pump by, for example, providing a fail-safe mechanism in consideration of a failure of the electric oil pump.
To overcome the foregoing problems and the like, it is conceivable to adopt a method in which, to move the vehicle by the drive torque of an electric motor, the electric motor is linked directly to an output shaft of the automatic transmission, as disclosed in Japanese Patent Application Publication No. 5-229350 (JP-A-5-229350). In the construction described in the publication No. 5-229350 (JP-A-5-229350), since the drive torque of the electric motor is transmitted to the driving wheels without intervention of the automatic transmission, it is not necessary to generate hydraulic pressure as mentioned above during the electric motor-driven travel. Thus, this construction eliminates the need to provide an electric oil pump.
However, in the construction described in the publication No. 5-229350, since the automatic transmission is not operated during the electric motor-driven travel, it is impossible to utilize the torque amplification effect of the automatic transmission during the motor-driven mode. Therefore, since the drive torque required during the electric motor-driven travel needs to be generated directly by the electric motor, the electric motor needs to be capable of generating large torque. Since the size of the electric motor and the magnitude of drive torque that the electric motor can generate are generally in a proportional relation, the foregoing technology of the publication No. 5-229350 has a problem of increasing the size of the electric motor and therefore increasing the size of the hybrid driving apparatus.